Method of making multilayer photoelectrodes and photovoltaic cells

ABSTRACT

A corrosion-resistant, multilayer photoelectrode for use in a photoelectrochemical cell and a process for producing said photoelectrode by preparing an effective layer of an insulator material on a base semiconductor and depositing a layer of conducting material on said layer of insulating material.

This is a division of application Ser. No. 434,603, filed Oct. 15, 1982.

BACKGROUND OF THE INVENTION

This invention relates to a stabilized multilayer photoelectrodeimmersed in an electrolyte in a photoelectrochemical cell, saidphotoelectrode comprises a base semiconductor, an effective layer ofinsulator material on said semiconductor and an effective layer ofconducting material on said insulator material. More particularly, thisinvention relates to a silicon based photoelectrode which comprisessilicon, an effective layer of oxide on said silicon, and an effectivelayer of conducting material on said oxide layer.

Photoelectrochemical cells are capable of generating direct electricalenergy as well as providing a means for storage of solar energy. Thebasic photoelectrochemical cell comprises a photoelectrode, acounterelectrode and one reduction-oxidation or redox couple in anelectrolyte. The simplest photoelectrode in the basic cell comprises asemiconductor with the front face illuminated by solar radiation and indirect contact with the redox-electrolyte solution which contains theredox couple. The back face of the semiconductor is connected to aninsulated wire, and a voltage is generated between the back face contactand the counterelectrode with electrons traveling in an external circuitformed by the wires between the two electrodes; and ions pass throughthe electrolyte between the two electrodes, completing the electricalcircuit. The junction between the redox-electrolyte solution andsemiconductor photoelectrode is a diode junction which acts much thesame as a p-n junction in a solid state solar cell; however, since thejunction between the electrolyte and semiconductor is a property of theinterface, its formation does not require the precise diffusion ofdopant material into the semiconductor which is usually important in asolid state device. The photoelectrochemical cell therefore hassubstantial differences from conventional solid state photovoltaiccells. These differences lead to important advantages over conventionalsolid state photovoltaic cells, such as the ability to use a broaderrange of materials for efficient cell operation, the ability to avoidthe constraints of lattice parameter matching between adjacent materiallayers, which is necessary for nearly all solid state photovoltaicdevices, and the ability to use small grain size semiconductor materialwithout any substantive decrease in solar conversion efficiency.

All these advantages of the photoelectrochemical cell have the potentialof leading to lower costs for production and storage of energy; butunfortunately, photoelectrochemical cells have some difficulties: (a)cell lifetimes are extremely short due to cell malfunctions caused byunwanted corrosion effects arising at the junction of the photoelectrodeand electrolyte solution, and (b) voltages and currents are often lessthan would be expected due to undesirable effects which arise, typicallyfrom flow of large currents the reverse of the desired direction (darkcurrents) or from recombination currents which diminish cell output andefficiency.

A number of publications have disclosed various attempts to preventcorrosion of the semiconductor photoelectrode in a photoelectrochemicalcell. One approach has been to utilize thin protective metal films,particularly gold and platinum, over the base semiconductor (see T.Skotheim, I. Lundstrom, and J. Prejza, J. Elec. Trochem. Soc., Vol. 128,1625 (1981)); however the films must be thin in order to permit light topass through to the semiconductor, and it is difficult to produceuniform, impermeable, thin metal layers and corrosion still occurs. Evenif the problem of corrosion protection were solved by a thin metal film,the voltage output, and consequently cell efficiency, is substantiallyreduced due to the ease of reverse dark current flow across thesemiconductor-metal film junction.

A second approach to stabilize the photoelectrode concerns the use of anultra-thin layer of a wide band gap oxide, typically TiO₂ or SnO₂, overthe base semiconductor (see A. J. Nozik, Second International Conferenceon Photovoltaic Conversion and Storage of Solar Energy, Aug. 8, 1978,Cambridge, England). Films such as TiO₂ are transparent but are alsoinsulating in character and if deposited with thickness sufficient toprotect against corrosion, the photogenerated charge carriers cannotpenetrate the insulating layer and thus the insulator layer preventsoperation of the cell. SnO₂ layers are also transparent to light, aremore corrosion resistant than TiO₂, and can be made conductive bydoping; however, SnO₂ has virtually no electrocatalytic activity (theability to enhance the kinetic exchange between electrons in theconducting layer and the redox reaction in the electrolyte solution).Electrocatalytic activity is quite important in driving the desiredredox couple reaction in the electrolyte solution. Therefore, unless anelectrocatalytically active layer is deposited on the SnO₂ layer, aphotoelectrochemical cell, which uses SnO₂ alone as a corrosionprotective layer, has an extremely low cell output.

A third approach to prevent photoelectrode corrosion concerns coating ofthe base semiconductor with an organic conductor layer (see R. Noufi, O.Tench, and L. F. Warren, J. Elec. Trochem. Soc. 127, 2310 (1980));however, severe problems are encountered in aqueous electrolytesolutions with the organic layers showing poor adhesion and at bestproviding protection for only a few days. A fourth corrosion protectionscheme concerns formation of derivatized layers over the basesemiconductor which are covalently bonded with the surface layer of thebase semiconductor but photoelectrode stability is maintained for onlyseveral days (see J. M. Bolts, A. B. Bocarsky, N. C. Palazzotto, E. J.Walton, N. S. Louis, and M. S. Wrighton, J. Am. Chem. Soc. 101, 1378(1979)).

Consequently, none of these references discloses a photoelectrode whichis stable for any time period in excess of several days, and whichproduces a high cell output with good efficiency. Accordingly, there isa need for an improved corrosion resistant photoelectrode which has along lifetime and shows improved photocell output and efficiency in theconversion of electromagnetic radiation to electrical power.

The general object of this invention is to provide an improvedefficiency, high output, corrosion resistant photoelectrode in aphotoelectrochemical cell. A more specific object of this invention isto provide an improved efficiency, high output, corrosion resistant,silicon based photoelectrode in a photoelectrochemical cell witheffective insulator and conducting layers on the base semiconductor.Other objects of this invention will be apparent to persons skilled inthe art from the following appended claims.

DESCRIPTION OF THE INVENTION

We have found that the objects of the invention can be obtained by aphotoelectrochemical cell comprising a multilayer photoelectrode whichcomprises a semiconductor, an effective layer of insulator material onsaid semiconductor, and an effective layer of conducting material on theinsulator layer. A preferred version of the invention comprises aphotoelectrochemical cell comprising a photoelectrode comprisingsilicon, an effective layer of silicon oxide or non-native oxide on saidsilicon, and an effective conducting layer on said oxide layer whereinsaid oxide and conducting layers are substantially continuous and pinhole free.

Briefly, this invention comprises a photoelectrochemical cell comprisinga multilayer photoelectrode which is resistant to corrosion by theredox-electrolyte solution present in a photoelectrochemical cell. Themost typical prior art redox-electrolyte/photoelectrode junctionconsists of a photoelectrode of a semiconductor alone or a semiconductorplus metal layer, and the photoelectrode is in direct contact with theredox-electrolyte solution. With this well known junction the corrosionreactions proceed rapidly. However, in the present invention, thephotoelectrode comprises a base semiconductor, an insulator layer overthe semiconductor, and a conducting overlayer which is in contact withthe redox-electrolyte solution.

In the simplest photoelectrochemical cell the diode characteristics andvoltage output of the cell are determined by differences in the chemicalpotential of the redox couple in the redox-electrolyte solution and thechemical potential of the semiconductor photoelectrode. In the instantinvention the properties of the diode junction and voltage output aredetermined by contributions from the insulator layer, the conductinglayer, and also the redox-electrolyte versus the base semiconductor,rather than just the redox-electrolyte solution versus thesemiconductor, as in the simple photoelectrochemical cell. Thus, thediode junction properties in this invention derive from a difference inchemical potential between the semiconductor potential and the sum ofthe other multilayer potentials including the redox couple in theredox-electrolyte solution. In the present invention the contribution ofeach of the portions of the photoelectrode in determining the netchemical potential as experienced by the base semiconductor arises fromthe following: (a) static charge and possibly the effective workfunction for the insulator, (b) work function for the conducting layer,and (c) the chemical potential for the redox couple in solution, asdetermined by the redox potentials (see M. A. Butler and D. S. Ginley,J. Matl. Sci. 15, 1 (1980)). These effects combine to produce anincreased chemical potential, and the greater the net difference inchemical potential between the summation of these components and thebase semiconductor, the larger the photovoltage produced by the cell.The dominating role of the redox-electrolyte in this potential energysummation is demonstrated in the special instance in which theconducting layer does not have the appropriate work function propertyfor the photoelectrochemical cell to function in air without theredox-electrolyte being present, i.e., the chemical potential does nothave the proper sign to permit operation. When the cell is immersed in aredox-electrolyte of the correct redox potential, the energy barrierbetween the electrolyte and photoelectrode is determined by the redoxcouple in the redox-electrolyte, and not by the conducting layer, andthus the cell now functions.

In a photoelectrochemical cell the base semiconductor functions eitheras a light-sensitive anode or a cathode depending on whether thesemiconductor is n- or p-type, respectively. The fact that there is ann-type or p-type semiconductor in contact with a system of different netchemical potential results in formation of a diode type region which isdepleted of majority carriers (electrons in n-type, holes in p-type) atthe semiconductor surface. This effect arises because within thisdepletion layer there exists an electric field which is able to separatespacially the electrons, which are optically excited by lightillumination to the conduction band, from the holes left behind in thevalence band. Thus, when the semiconductor is illuminated with photonsof energy greater than the band gap of the base semiconductor, electronsare excited into the conduction band. Those electronhole pairs createdin the depletion layer are separated by the electric field in thedepletion layer before the holes and electrons can recombine. Themajority carriers then pass through the base semiconductor to the backface ohmic contact and are drawn off as a current by an insulated wire.In the multilayer photoelectrode the minority carriers are drawn acrossthe insulator layer adjacent the base semiconductor by tunneling throughthe insulator layer, and are passed through an outer conducting layer tothe interface with the redoxelectrolyte and react with the redoxcomponents in the redox-electrolyte.

The present invention in one aspect is operable as a regenerativephotoelectrochemical cell wherein only one redox couple is present inthe redox-electrolyte; thus, for example, reduction will occur at thephotoelectrode while the inverse oxidation will take place at thecounterelectrode. Consequently, in the reversible iodine-iodide system,the reaction I₂ +2e⁻ →2I⁻, takes place at the cathode; and the oppositereaction, 2I⁻ I₂ +2e⁻, occurs at the anode with e⁻ flow in the externalcircuit and I⁻ flow in the redox-electrolyte between electrodes.Therefore, in such a system there is no net chemical change, and thepower produced by the photoelectrode is extracted via the externalelectrical load.

The presence of a redox-electrolyte in a photovoltaic cell not only actsto form a convenient diode junction, as in the regenerative cell justdescribed, but also readily permits the storage of electrical power.Using conventional photovoltaic cells, external batteries need to beutilized to store energy. Similarly in the case of regenerativephotoelectrochemical cells external storage batteries are necessary tostore energy. Energy can, however, be stored internally in a particularvariety of photoelectrochemical cell which has at least two redoxcouples. Therefore in another aspect of the instant invention, thestorage-type cell includes a photoelectrode immersed in a firstredox-electrolyte solution containing a first redox couple (such asiodine-iodide) and a counterelectrode, typically platinum, immersed in asecond redox-electrolyte (which can be the same as the firstredox-electrolyte) containing a second redox couple (such asbrominebromide). The two redox couples are separated from each other byan ion conducting membrane which allows maintenance of charge balancebetween the two compartments by flow of a nonactive species (forexample, H⁺, Na⁺, K⁺, Cl⁻) present in the redox-electrolytes. Thedriving force for the storage cell is the difference in the free energyof reaction between the two different redox couples taking into accountthe relative concentrations, and energy is stored by each redox coupleby driving the redox reactions with the photoenergy produced fromillumination of the photoelectrode. Alternatively, rather than allowingthe reduction-oxidation reactions to take place, the charge carriers arecollected at the surface of the electrodes by metallic contacts, and thecharge is removed to perform useful work directly.

For the instant invention a storage-type redox cell which stores powerby oxidation-reduction reactions, has the following operation: (a)quanta of electromagnetic radiation enter the semiconductor andhole-electron pairs are formed; (b) electrons tunnel through theinsulator layer; (c) the electrons or holes are captured during a rapidchemical reaction at interfaces between the redox-electrolyte andconducting layer, performing chemical reduction or oxidation reactions.The energy that is stored by the oxidation-reduction reactions is thentapped to draw off electrical power to an external load by running thereaction in a reverse direction, typically by using the conducting layerof the photoelectrode to collect current, or another current collectingelectrode coupled with the counterelectrode.

A fundamental requirement for any commercially feasiblephotoelectrochemical cell is a photoelectrode which has a longoperational lifetime. The limiting factor of the photoelectrode lifetimeis usually photoelectrode corrosion, particularly anodic corrosion.Photoanodic corrosion is much more rapid than photocathodic corrosionbecause photocathodes are protected against oxidative corrosion by thephotogeneration of electrons which arrive at the basesemiconductor/redox-electrolyte interface. While reductive corrosion ispossible, it is less likely to occur because reduction products (forexample, hydrides) are less readily formed than oxidative products. Thispresent invention overcomes both anodic and cathodic corrosiondifficulties by means of a multilayer photoelectrode configuration. Thepreferred photoelectrode configuration comprises an appropriate basesemiconductor, an insulator layer on the base semiconductor, and aconducting layer on the insulator layer wherein both the insulator andconducting layers are substantially continuous, pin hole free layers.

The base semiconductor absorbs light which generates hole-electronpairs. The insulator layer acts as an insulator to diminish any reversedark current flow across the photoelectrode, while still allowingtunneling of carriers in the forward direction, and also preferably isable to alter surface defects on the semiconductor to abate electronicstates which have deleterious effects on solar cell efficiency. Theouter conducting layer of the photoelectrode should be able to conductefficiently the photogenerated electrical carriers and preferably alsoexhibit electrocatalytic properties.

BASE SEMICONDUCTOR

The base semiconductor preferably has a valence to conduction bandoptical gap energy such that a large portion of the electromagneticspectrum incident on the photoelectrode semiconductor will result increation of hole-electron pairs by photoexcitation of electrons from thevalence to the conduction band. In the case of an electromagneticspectrum from our sun, a desirable energy gap for the semiconductor isapproximately 0.8 to 2.0 eV and preferably about 1.3 eV. Typicalsemiconductors and band gaps (in parentheses) which would be suitablefor solar spectrum conversion include, but are not limited to, singlecrystalline, polycrystalline, and amorphous forms of: silicon (1.2 to1.7 eV), GaAs (1.3 eV), Ge (0.78 eV), B₄ C (˜0.5 eV), InP (1.25 eV),AlSb (1.6 eV), InSe (1.0 eV), ZnSe (2.6 eV), Ca₂ Si (0.9 eV), Ca₂ Sn(0.9 eV), GaSb (0.78 eV), GaP (1.8 eV), CdSe (1.74 eV), CdTe (1.45 eV),Cu₂ O (2.1 eV), CuInS₂ ( 1.4 eV), CdSnP₂ (1.2 eV), CuAlSe₂ (2.7 eV),CdSnAs₂ (0.3 eV) organic semiconductors, Such as copper phthalocyanine(˜2 eV), amorphous carbon (0.9-2.1 eV), and mixtures thereof.

It is also feasible to prepare photoelectrodes having at least twosemiconductor layers in order to complement each other to utilize moreof the incident electromagnetic spectral energy. For example, a firstlayer of Si has an optical gap of approximately 1.2 eV, and a secondlayer of Ge with a gap of 0.7 eV would absorb the longer wavelengthradiation passed through a silicon layer of suitable thickness. The twosemiconductors, together, would then convert a greater portion of theincident solar spectrum than just one silicon or germanium layer alone.The two, or more, semiconductor layers also should have some separatingstructure which would alleviate formation of structural defects due tolattice mismatch between the semiconductors. As is well known in theart, the intervening structure could be a chemically graded layer or asuperlattice layer structure of the type disclosed in Blakeslee, U.S.Pat. No. 4,278,474, which is incorporated by reference. It is alsofeasible to have two or more base semiconductor regions with aredox-electrolyte solution interposed between the base semiconductorsand thereby avoid difficulties associated with lattice mismatch.Further, such a redox-electrolyte solution is useful not only as amedium for a redox-couple reaction in an electrolyte solution, but alsoas a heat exchange fluid for utilization of energy converted intothermal energy by a photoelectrochemical cell.

The semiconductor preferably has an energy band structure relative tothe insulator layer, the conducting layer, and the redox couple in theredoxelectrolyte which results in obtaining the maximum power from thephotoelectochemical cell. In an n-type semiconductor the Fermi level isnear the conduction band and one way to create a large potential energybarrier height at the junction between the semiconductor and insulatorlayer is to have: (a) an outer conducting layer with a work functionhigher than the base semiconductor work function, and/or (b) a redoxcouple in the redoxelectrolyte with a chemical potential lower than thebase semiconductor work function, and typically an oxidizing-typeredox-electrolyte will satisfy this requirement. Herein the term "workfunction" is the definition well known to one of ordinary skill in theart, that is, the energy required to remove an electron from the Fermilevel to vacuum with the particular material defined to be in isolation.

Since the volume of redox-electrolyte solution is much greater than thevolume of the very thin outer conducting layer, the dominant influenceon the potential energy barrier height usually will be the differencebetween the semiconductor work function and redox-electrolyte potential,provided the conducting layer and redox-electrolyte are effectivelycoupled by rapid redox reactions at the photoelectrode surface.Effective coupling means the Fermi level of the conducting layer isequal or comparable to the chemical potential of the solution, asituation aided by rapid kinetic exchange between the electrons in theconducting layer and the solution species. This effective coupling ofelectrolyte and conducting layer is typically accomplished by thepresence of electrocatalytic properties in the conducting layer or byaddition of a small amount of electrocatalytic material on theconducting layer. In the absence of effective electrocatalyticproperties the potential energy barrier height of the photoelectrodewill be dominated by the difference in work functions of the conductinglayer and base semiconductor.

Additional interactions at the base semiconductor/insulator layerinterface also will affect the photovoltaic output voltage. In thepresence of fixed charges, which arise from chemical defects in theinsulator layer, an additional energy barrier is induced in thephotoelectrode. For example, in an n-type base semiconductor, if defectsin the insulator layer have a negative virtual charge, a positive chargeis induced in the base semiconductor which results in a potential energybarrier at the base semiconductor-insulator interface. Typical chemicaldefects which are capable of inducing such barriers includenon-stoichiometric defects in the insulator, such as interstitialsand/or vacancies on the anion or cation sublattices and substitutionaldefects arising from the presence of aliovalent ions.

INSULATOR LAYER

In the instant invention the layer over the front face of the basesemiconductor in the photoelectrode is an effective insulator layer. Inorder to be an effective layer the insulator layer should be thin enoughthat neither the transmission of electromagnetic radiation, nor thetransport of charge carriers in the desired direction through the layeris significantly reduced. Upon illumination of the photoelectrodecomprising an n-type base semiconductor, electrons are transferred fromthe redox-electrolyte into the photoelectrode accompanied by anoxidation reaction of species in the redox-electrolyte. The electronspass through both the outer conducting layer and the insulator layerinto the valence band of the n-type base semiconductor. The photoexcitedelectrons from the base semiconductor pass through the back contact viaan ohmic contact to an external circuit, through the counterelectrode,and into the redox-electrolyte to complete the circuit.

The structure and properties of an effective insulator layer shouldtherefore permit the passage of electrons from the face of thephotoelectrode which is in contact with the redox-electrolyte, to theohmic contact on the opposite face of the base semiconductor. The netcurrent output is diminished by any dark current flowing in the reversedirection. Typically an oxide-type insulator layer of about 10 to 25Angstroms thickness permits tunneling of electrons from the front faceconducting layer across the insulator layer into the base semiconductorvalence band wherein states have been left empty by the photoexcitationof electrons to the conduction band. The reverse dark current flow ofelectrons from the conduction band of the base semiconductor into theconducting layer is substantially reduced because electrons in then-type base semiconductor are displaced from the insulator/semiconductorinterface by the thickness of the charge depletion layer, typicallyhundreds of Angstroms. Consequently, the electrons have to tunnel thisadditional depletion thickness, and tunneling probability diminishesrapidly with tunneling distance; therefore, the reverse current flow islowered by the presence of the insulator layer. This has the effect ofincreasing both the observed cell voltage and current.

In order to fulfill the above preferred operating characteristics for aneffective insulator layer, the thickness of the layer should bewell-controlled, and are preferably substantially integral or pin holefree to be most effective. This is typically accomplished by havinginsulator layers of at least 5 Angstroms thickness. However, layersthicker than about 25 to 50 Angstroms do not allow adequate electrontunneling, the insulator acts as a blocking resistance, and the cellcurrent drops. Various parameters characterize the precise thickness fora given configuration and materials which will yield effective insulatorproperties, while permitting tunneling in the desired direction.Effective thicknesses can be derived from the following works which areincorporated by reference: A. G. Milnes and D. L. Feucht,Heterojunctions and Metal-Semiconductor Junctions, Academic Press, N.Y.(1970); and S. Roy Morrison, Electrochemistry at Semiconductor andOxidized Metal Electrodes, Plenum Press, N.Y. (1980). In the moregeneric sense the fulfillment of the criterion for an effectiveinsulator layer represents an improvement for a multilayer deviceconfiguration whether as part of a photoelectrode in aphotoelectrochemical cell or, for example, as part of an MIS(metal/insulator/semiconductor) solid state device which for purposes ofthis disclosure can be defined as a photoelectrode in air (see Examples9, 10, 13, and 21).

For the special case of native oxides on the base semiconductor, growthof native oxides to an effective insulator layer thickness can take aslittle as half an hour in air (silicon oxide on n-type silicon) or maynever be achieved in air at ambient temperatures (silicon oxide onp-type silicon). Example 7 illustrates the substantial effect that timein air has upon the photoelectrochemical cell current output for ann-type, silicon base semiconductor. Exposure to air for about 2 minutesresults in a cell current of about 21 ma/cm². An optimum air exposuretime is approximately 12 to 25 minutes with a cell output of about 28ma/cm², and a rapid decline to 8 ma/cm² cell output after an airexposure time of 50 minutes. Furthermore, for such materials as GaAs thenative oxides have physical properties which are inadequate to provideeffective insulator properties for the photoelectrode (see T. Hariu etal., Appl. Phy. Lettr. 32, 252 (1978)). Therefore, preparation of aphotoelectrode with an effective insulator layer of optimal thicknessdemands substantially more than formation of a native oxide layer ofindeterminate thickness and unknown effectiveness. Moreover, non-nativetype insulators, which are deliberately fabricated on the basesemiconductor, can constitute the insulator and provide for improvedcell performance compared to the natural oxide. Examples are thecombinations of Si/Ta₂ O₅, Ge/Al₂ O₃, CdS/Si₃ N₄. The use of Ta₂ O₅ onSi instead of SiO₂ on Si results in an increased cell voltage (seeExample 18). The thin insulator layer is, for instance, deposited on thebase semiconductor using chemical vapor deposition, electron beamdeposition, or sputtering techniques. Combinations of native andartificial insulators are also possible.

A second type of non-native type insulator is a doped insulator, inwhich the deliberate introduction of impurity species is used to modifythe fixed insulator charge and/or modify the surface states. An exampleis the alteration by doping of a native silicon dioxide with aluminum(see Example 5), achieved by coating the surface of the silicon basesemiconductor with a small amount of an aluminum salt prior tooxidation. Another example is yttria doped zirconia deposited as theinsulator layer with n-silicon as the base semiconductor (Example 21).Further, this type of insulator layer represents an improvement withmore generic applications in devices such as MIS configurationsdescribed previously.

This multilayer photoelectrode (base semiconductor/insulator/conductinglayer/redox-electrolyte) may be contrasted to a Schottky barrier devicein which no intervening, integral insulator layer is present. Generally,in a Schottky device, the reverse dark current and forward current aremuch more nearly equal which would yield an undesirably small netvoltage output for a Schottky configuration.

For our purposes, an insulator is defined to include all materialswhich, upon connection to a base semiconductor and a conductingoverlayer, have the band gap of the insulator material encompassing theenergy window through which the reverse dark current flows. For example,classical oxide insulators such as Al₂ O₃, SiO₂, MgO, MgAl₂ O₄, Ta₂ O₅,B₂ O₃, ZrO₂, TiO₂, rare earth oxides such as Y₂ O₃, and lanthanides suchas CeO₂ and La₂ O₃, have wide band gaps which readily accommodate thisdefinition. Other insulators can include nitrides, such as Si₃ N₄ or BN,carbides such as SiC, and halides such as NaF or PbCl₂. All of the abovemay be either undoped, or doped so as to function as desired. Alsoincluded are selectively matched semiconductors with suitable widths forthe band gap and even appropriate degenerately doped semiconductors,which include n⁺ with a p-type base semiconductor, p⁺ with an n-typebase semiconductor. Also these conditions can be met for n⁺ with n-typeand p⁺ with p-type, provided the degenerate electronic states or thebands are not in the energy window of the reverse dark currents. Certainappropriate narrow band metals, such as MnP, also fulfill the aboverequirement, i.e., the partly filled bands are narrow enough such thatthere is a band gap of sufficient width to encompass the dark currentenergy window (see J. B. Goodenough, D. H. Ridgley, and W. Newman, Proc.Intl. Conf. Magnetiam, Nottingham, Int. Phys. and Phys. Soc., 1964).Also included are insulating organic materials such as polyethylene.

The insulator layer preferably also exhibits electronic and structuralproperties such that the number of detrimental interfacial electronicstates at the base semiconductor/insulator junction is diminished by thepresence of the insulator layer. For example, the surface of a typicalsemiconductor exhibits some atoms which have incomplete covalent bonds,i.e., dangling bonds. These dangling bonds have characteristicelectronic orbitals which are at an energy level within the energy gapestablished between the valence and conduction bands of the basesemiconductor. These intragap electronic states often assist in therecombination of electrons and holes created by the photoexcitationprocess, resulting in the loss of charge carriers and diminishment ofoverall photocell efficiency. Therefore, the insulator layer preferablyacts to compensate, or to fulfill, the bonding needs of the incompleteor dangling bonds of the surface atoms. Compensating these bonds removesthem from the energy band gap region and eliminates electronic stateswhich otherwise would assist in the recombination of electrons andholes. The net result is an increase in both the carrier current and incarrier lifetimes.

In addition to dangling bond-type surface electronic states in theenergy gap, other typical surface electronic states comprise: (a) grainboundary electronic states in a polycrystalline base semiconductor, (b)electronic states associated with dislocation line defects, and (c)electronic states formed by bonds between the base semiconductor andanother element or compound which are weaker bonds than the self-bond ofthe base semiconductor, and therefore still lie in the band gap.

Typically, insulators which act to alleviate intergap states take theform of an oxide or nitride layer of the base semiconductor layer, sincethese bonds are usually at least as strOng as the base semiconductorcovalent self-bond energy. Examples include a silicon oxide or siliconnitride layer over an underlying silicon layer, or In₂ O₃ on an InP basesemiconductor. Compensating layers which do not have a common elementwith the base semiconductor can be beneficial as discussed above. Thesecompensating layers include, but are not limited to, boron nitride,aluminum oxide, tantalum oxide, tin oxide, and titanium dioxide.Interfacial states also are abated by annealing in a hydrogen and/orfluorine gas atmosphere which acts to compensate defects such asdangling bonds (see Example 9).

Intragap electronic tunneling states also are capable of being createdby various defects, such as grain boundaries, dislocations, impurities,and other free surfaces. Tunneling states are intragap states positionedin or near the depletion layer and energetically located near thepositions of the valence and conduction band levels which define theenergy gap. Some of these tunneling states lead to enhanced transport ofmajority carriers across the depletion layer. The flow of majoritycarriers across the depletion layer is undesirable in those cases inwhich one is seeking to minimize leakage currents arising from energylevels near the valence band or conduction band; for example, since thedevice is intended to function as a diode, (allowing current flow in onedirection only) and the reverse dark current or leakage current isopposite the intended direction of current flow, then the tunnelingstates result in detrimental current and voltage outputs.

Higher cell voltages are achieved by doping the insulator to createcharges in the insulator which induce complimentary opposite charges inthe semiconductor, thus enhancing the voltage barrier. For p-type basesemiconductors, such uncompensated virtual positive charges in theinsulator will be compensated in the layers of the base semiconductoradjacent to the insulator, resulting in enhanced band bending and largerbarrier heights. Pronounced band bending will produce an inversion toform a degenerate n⁺ region near the interface. In the first class ofexamples, the virtual positive charges are extrinsic dopant ions, suchas interstitial Na⁺ in SiO₂, or extrinsic substitutional aliovalentdopants such as Nb⁵⁺ in ZrO₂, Al³⁺ in MgO, Mg²⁺ in NaCl, F⁻ in Al₂ O₃,or O²⁻ ions in Si₃ N₄. In the second class of examples, the virtualpositive charges are intrinsic defects such as Ni³⁺ ions in nickeloxide, or excess silicon in silicon dioxide.

For n-type base semiconductors, uncompensated virtual negative chargesin the insulator will be compensated in the adjacent layers of the basesemiconductor, resulting in enhanced band bending in the oppositedirection compared to above. Large barrier heights will result frominversion to produce degenerate p⁺ layers near the interface. In thefirst class of examples, the virtual negative charges are extrinsicsubstitional aliovalent dopants such as Al³⁺ in SiO₂ (see example 5),Y³⁺ ions in ZrO₂ (see Example 21), Mg²⁺ ions in Al₂ O₃, Li⁺ in MgO, Ti⁴⁺in Ta₂ O₅, N³⁻ ions in SiO₂, O²⁻ ions in NaCl, or carbide ions in Si₃N₄. In the second class of examples, the virtual negative charges areintrinsic substitutional aliovalent defects such as Ta⁴⁺ ions in Ta₂ O₅(see example 18), Ti³⁺ ions in TiO₂, Fe²⁺ in Fe₂ O₃ , In⁺ in In₂ O₃, Zn⁺in ZnO, or excess interstitial ions such as F⁻ in CaF_(2+x) or O²⁻ inUO_(2+x).

In addition to enhancing the barrier heights, and thus the cellvoltages, the minority carrier current through the doped insulator willbe enhanced, and thus the cell current and voltage improved by thepresence of dopant energy levels in the insulator which are within theenergy window for minority carrier tunneling, thus allowing thickerinsulators to be used. In the case of non-native oxides, several atomiclayers of native oxide may also exist between the base semiconductor andthe non-native insulator so as to reduce the surface state density andalso assist in lattice matching of adjacent layers. Such improvedmultilayer diodes incorporating doped insulators are adaptable as photocells in air when two electrical contacts are made to the front and backfaces.

Conducting Layer

In a photoelectrochemical cell the photoelectrode layer adjacent theredox-electrolyte solution is an effective conducting layer. In order tobe an effective conducting layer, the layer should be a good conductorof the electrical carriers, have a structure which will provideeffective corrosion protection, preferably exhibit sufficientelectrocatalytic properties to drive the redox reactions at an adequaterate, and should be thin enough to allow passage of a sufficient amountof electromagnetic radiation to permit efficient operation of thephotoelectrochemical cell. If, however, the base semiconductor isilluminated from the opposite or back face, as in specialconfigurations, then the thickness of the conducting layer on the frontface is not limited by its transmissivity, and thick layers could beused. Provided this layer is of the appropriate thickness andsufficiently uniform to be substantially an integral layer, the layerwill act to inhibit corrosion of the photoelectrode, will normally allowpassage of light into the base semiconductor, will have sufficientelectrical conductivity to enable the photo-induced charge carriers totraverse the layer without any significant carrier loss, and havesubstantial electrocatlytic character (see Modern Aspects ofElectrochemistry, No. 12, Eds. J. D. M. Buckris and B. E. Conway,Plenum, 1977, pp. 183-266). Typical preferred materials for thisconducting layer include, but are not limited to: a noble metal likepalladium, platinum, (it is preferable to anneal the photoelectrodeconfiguration either during or after deposition of the platinum layer onthe insulator layer (see Examples 1 and 19)) iridium, ruthenium, ortantalum, platinum silicide, LaB₆, and conducting amorphous carbon;semimetals, such as MoSi₂, WSi₂, and CrB₂ ; semiconducting chalcogenidesor metallic pnictides, such as MoS₂ and TiN, respectively; degeneratelydoped semiconductors, such as SnO₂ doped with indium; doped oxides, suchas WO₃, MoO₃, or TiO₂ ; and conducting organics, such as polypyrrole.

In those particular cases where the conducting layer does not havesufficient electrocatalytic activity to drive the redox reactions toproduce a reasonable output for the cell (see Example 14), anelectrocatalytically effective material (such as a noble metal, Ni,RuO₂, Mo, Ta, iridium tantalate, a nickel-molybdenum alloy, graphite oramorphous carbon and mixtures thereof) can be deposited over any generalconducting layer of low electrocatalytic activity (see Examples 1, 13and 22). Therefore in these particular cases an effective conductinglayer for purposes of this disclosure becomes a general conducting layerregion covered by a thin layer of electrocatalytically active material,such as a noble metal. Preferably the conducting layer is a thin layerof about 10 to 200 Angstroms of platinum, or an electrolyticallyinactive conducting layer such as non-stoichiometric SnO₂ or doped SnO₂(of about 30 to several hundred Angstroms thickness) with several tensof Angstroms of platinum on the inactive conducting layer. The mostpreferred conducting layer is 10 to 50 Angstroms of platinum. (Asexplained in Example 1, all thicknesses determined in the evaporatorunit by a deposition monitor are approximate and can be in error by afactor of two but are correct relative to one another.)

Electrolyte Solution

The electrolyte is preferably aqueous, but other redox-electrolyteswhich conduct ionic currents and contain a redox potential appropriatelymatched to the band structure of the multilayer photoelectrode are alsosuitable redox-electrolytes. Nonaqueous electrolytes which are suitableinclude polar solvents, such as ethanol and methanol, having appropriateionic conductors in the solvent. In the typical configuration for aphotoelectrochemical cell wherein light passes through theredox-electrolyte to the photoelectrode, the redox-electrolyte also ispreferably substantially transparent. In an aqueous redox-electrolytepreferred redox couples are I₂ /I⁻ and Br₂ /Br⁻ with the work functionof the base semiconductor matched with the redox potential of theredox-electrolyte. This is, however, not necessary in those cases wherethe fixed charge of the insulator layer determines the barrier potentialenergy of the photoelectrode. Another suitable redox-electrolyte is afused salt, such as aluminum chloride with n-butyl pyridinium chloride.This redox-electrolyte is particularly preferred when n-type GaAs is thebase semiconductor. The preferred redox couple in this case is irondicyclopentadienyliron dicyclopentadienyl chloride. In the case of anorganic redox-electrolyte an example is acetonitrile and iodine/iodideas the preferred redox couple. In a particular type ofphotoelectrochemical cell one or all of the redox components can begaseous, rather than liquid, as for example in a photoelectrolysis cell.

The ability to use a redox-electrolyte to form a diode junction with themultilayer photoelectrode also enables the preparation of aphotoelectrochemical cell which is inoperable in air without theredox-electrolyte. In this case, a contact wire is joined to theconducting layer, and the voltage and current are recorded between thiswire and the normal back face contact, enabling the photoelectrode to betested in air. For example, when using p-type silicon as the basesemiconductor, it is necessary to have a low work function, conductinglayer over the base semiconductor in order for the photoelectrode tohave the proper potential energy barrier relationships to be functionalin air. However, appropriate low work function conductors, such as Cr,Cu, Al, and Mg, all quickly corrode when placed in a redox-electrolytesolution and thus render the photoelectrode inoperative. Due to thepresence of a redox-electrolyte in this invention, a high work function,conducting layer such as Pt can be placed over the base semiconductor,and a low work function redox-electrolyte solution (such as a reducingsolution of V³⁺ /V²⁺ or S²⁻ /S_(n) ²⁻ (polymeric form)) can be used toproduce an operative photoelectrode using p-type silicon (see Example14). Without the low work function redox-electrolyte this photoelectrodewould be inoperative in air since a high work function, conducting layeris over a p-type base semiconductor, and the potential energy barrierwould not permit current flow.

Ohmic Contacts

Completion of the electrical circuit of the photoelectrochemical cellrequires at least a wire from the counterelectrode joined to the backface of the base semiconductor of the photoelectrode by means of anohmic contact. The ohmic contact to the base semiconductor may be formedby a variety of ways known to those of ordinary skill in the art.However, in this invention a new, improved contact to n-silicon waspreferably formed by depositing on the back face of the basesemiconductor the following layers: a first layer of phosphide,arsenide, or antimonide, followed by a second metal layer, and then afinal conducting layer. The phosphide, arsenide, or antimonide layer isa compound with a metal cation which is capable of forming a conductingmetal oxide compound with any underlying oxide compound (such as nativeoxides). Examples of cation candidates include Ti, Zr, Hf, V, Nb, Ta,and other metals enumerated in the publication by A. T. Howe and P. J.Fensham, Quart. Rev. 21, 507 (1967) which is incorporated by reference.In a more generic sense the second metal layer is not required if thefirst layer of phosphide, arsenide, or antimonide is thick enough totake up all the oxygen from any underlying native oxide or depositedoxide layer. The final conducting layer is a conducting material stablein air, and not reactive with the underlying layer, for example, Pt, Pd,Au, W, Ta, MoSi₂, doped SnO.sub. 2, doped oxides, such as WO₃, orconducting organics such as polypyrrole. The most preferred embodimentcomprises layers of vanadium phosphide, titanium, and platinum on theback face of the base semiconductor. Since the metal layer on thephosphide, arsenide, or antimonide layer takes up oxygen to form aconducting oxide, an ohmic contact is formed without the necessity ofremoving any oxide layers from the base semiconductor surface. One ofthe main difficulties in making ohmic contacts to any semiconductor isinvariably the presence of native oxides, or other unwanted oxide layerson semiconductors which have been exposed to an oxidizing atmosphere. Inthe present invention for the particular case of n-type silicon as thebase semiconductor, the silicon had accumulated approximately 10Angstroms of native oxide, and the deposition of vanadium phosphide,titanium, and platinum layers resulted in formation of suitable ohmiccontacts of several ohms resistance (see Examples 1 and 16).

In the production of an ohmic contact in the instant invention thevanadium phosphide layer performs two important functions: (1) itprovides a source of phosphorous to form the n⁺ degenerately doped layerof silicon, and (2) it provides a source of vanadium to reduce surfaceoxides and form a conducting vanadium oxide layer. Any excess vanadiumphosphide will also be conducting since the material is a metallicconductor, and as indicated previously, any excess oxygen isaccommodated by reaction with the reducing metal layer between thevanadium phosphide and the protective outer metal layer to form aconductive oxide.

In the present invention other suitable materials for making ohmiccontacts to n-type semiconductors are metal hexaborides on thesemiconductor and any air stable conducting layer on the hexaboride.Preferably the layer on the base semiconductor is one of the lanthanideborides, and most preferably is lanthanum hexaboride. Metal hexaborideswith a 3 oxidation state and some with a +1 oxidation state exhibitmetallic conductivity. The metal hexaborides provide a low work functionohmic contact to n-type semiconductors in general and also can be usedas an ohmic contact to photovoltaic Schottky barrier devices composed ofp-type semiconductors. Further, most organic semiconductors are p-typeand thus require a low work function metal for a Schottky barrierphotovoltaic device. In a photoelectrochemical cell, lanthanumhexaboride provides a material transparent to visible and ultravioletlight, has suitable conductivity, forms an ohmic contact to the basesemiconductor, and shows good resistance to corrosion which is essentialfor long-term stability in a photoelectrochemical cell with an n-typebase semiconductor as the photoanode. The ohmic contact is then formedby deposition of the hexaboride on the n-type base semiconductor,followed by deposition of a metal layer on the hexaboride, such asplatinum, and then silver paste and a copper wire are attached to themetal layer. A preferred configuration in the instant invention is anintegral layer of at least 20 Angstroms of LaB₆ and several hundredAngstroms of platinum (see Example 17).

ADDITIONAL EMBODIMENTS

The multilayer structure in the generic sense constitutes a diode inwhich an internal voltage barrier allows current flow more in onedirection than the other, and such a diode is incorporated in thephotoelectrode. Other devices which incorporate such a diodeconfiguration are: (a) a photocell in which electrical connections aremade directly to the back and front of the diode structure, (b) aphotodiode, in which the diode properties are dependent upon thecharacteristics of the incident light spectrum, (c) a dark diode actingas an electrical rectifier, (d) a photocapacitor device, (e) a darkcapacitor device, and (f) an electrode in which the diode barrier heightdepends on the redox potential of the medium surrounding the diode, andin which the potential energy barrier height is measured directly (orindirectly as in a field effect device), to indicate redox potentials ofthe surrounding fluids.

More particularly, there are several other particular embodiments whichthe present invention incorporates using the same basic principles ofoperation as the photoelectrochemical cell described above. One of thesedifferent embodiments comprises a photoelectrochemical cell with thephotoelectrode undergoing illumination from the direction of a dry backface, rather than radiation passing through the redox-electrolytesolution and striking the photoelectrode front face. Such aconfiguration has the advantage that the incident radiation does nothave to pass through the redox-electrolyte solution. Therefore, thickerfront face conducting layers can be used for the photoelectrode; and theredox-electrolyte solution can be opaque to the incident radiation,thereby allowing a broader choice of redox-electrolyte solutions.Furthermore, the photoelectrochemical cell components are capable ofbeing enlarged on the front face since these components need not betransparent to the radiation. For example, the counterelectrode may beattached to the front face with appropriate insulation, membranes can bemounted, and various current-collecting electrodes and monitorsattached. However, the back, dry face ohmic contact and the basesemiconductor should be sufficiently transparent to allow absorption ofmost of the incident radiation by the base semiconductor. Suitable meansto fulfill these requirements include using current collection by a finegrid of opaque metal or using a transparent conductor as the currentcollector, such as doped SnO₂. If the base semiconductor thickness istoo large, current collection efficiency will diminish due torecombination of holes and electrons before the carriers diffuse throughthe base semiconductor to the front face diode region where holes andelectrons are separated. For example, doped SnO₂ current collectors canbe deposited on a protective glass plate with the base semiconductordeposited on the doped SnO₂, followed by a layer of insulator, and thena conducting material layer adjacent the redox-electrolyte solution.During actual use such a configuration can be mounted on the outside ofa building roof with light striking the back face covered by the glassplate.

In another configuration of the present invention, light is alsoincident on the back face of the photoelectrode. The back face materialsneed to be substantially transparent, and light then passes through tothe base semiconductor with some light absorbed and some passing on tothe conducting layer. The conducting layer provides a reflective surfacewhereby the light is reflected back into the base semiconductor, andtherefore some of the light passes back through the base semiconductorwith twice the path length available for light to be absorbed. Thisconfiguration utilizes the reflective properties of the conductinglayer, and the thickness of the base semiconductor can be reduced.

In another aspect of the present invention situations occur in which oneof the electrodes, counterelectrode or photoelectrodes, is replaced bythe other type of electrode. For example, a counterelectrode can bereplaced by a photoelectrode such that the band gaps of the tworesulting photoelectrodes are different. The two photoelectrodes areseparated by a redox-electrolyte, or two redox-electrolytes with anintervening membrane. Light components of high energy are absorbed bythe first photoelectrode, and the remaining lower energy light passes onto be absorbed in the second photoelectrode. Using the ion conductingmembrane to separate two solutions of different redox battery, the twophotoelectrodes can be used to charge up the redox battery. Theconductor layer of the multilayer configuration acts as a currentcollector as well as functioning as an essential part of thephotoelectrode. Electrical connections to this conductor layer of eachphotoelectrode are used for the following:

(a) direct collection of electrical output from the cell rather thanbattery charging when illunination is adequate,

(b) direct collection of electrical output when the battery is fullycharged, and

(c) collection of electrical power during discharge of the battery andno additional counterelectrodes are necessary.

An additional aspect of the present invention is a photoelectrolysiscell, (which is a particular type of photoelectrochemical cell),comprising one or more gas phase redox components which replace thesolution redox components of the photoelectrochemical cell with theprovision that ionic conduction is still accomplished between the twoelectrodes of the cell. In one configuration water, HBr, or HI areelectrolysed between a photoelectrode and a counter electrode immersedin an electrolyte containing the reactants, and hydrogen gas plus theother decomposition products are produced. In another configuration thephotoelectrolysis of water is done using two plate photoelectrodeshaving different optical band gaps, arranged to absorb successively thelight components. This basis configuration is retained in anotherexample except that the two photoelectrodes are connected by a sheet ofporous material which conducts protons from one electrode to the other.Water vapor is passed through the porous separator and is converted ateach photoelectrode face into either hydrogen or oxygen. Anotherphotoelectrolysis configuration comprises two photoelectrodes ofdifferent band gap connected by a proton conducting medium, and watervapor is passed over the outside face of each photoelectrode withprotons reaching the outside faces of the photoelectrodes through poresor holes in the photoelectrodes. Photoelectrolysis proceeds and produceshydrogen and oxygen gas.

EXAMPLE 1

In the examples that follow the starting materials for the basesemiconductors are silicon single-crystals, (100) orientation,approximately 1 inch diameter wafers supplied by Atomergic ChemicalsCorporation. These wafers were cut into smaller sizes for preparation ofphotoelectrodes. The crystal in this example was n-type, phosphorusdoped, with a resistivity of 1.0 ohm centimeters. The polished face ofthe crystal was placed face down and a drop of etchant placed on theunpolished face which was etched twice for two minutes each time with a10 volume percent HF/36 volume percent NH₄ F aqueous solution and thenwashed with distilled water for 30 seconds. The crystal was then driedunder ambient conditions with a flowing nitrogen gas stream.

Back ohmic face contacts on the base semiconductor were fabricated asfollows: A platinum slug, titanium slug, and vanadium phosphide pelletwere placed in separate carbon crucibles, and all three of the cruciblesplaced in a Balzers electron beam evaporator. The evaporator vessel waspumped down to a pressure of 10⁻⁵ millibars pressure. The aluminumsubstrate holder to which the silicon crystal was attached was heated bya resistive heater positioned in the top of the vacuum chamber. Thetemperature was raised to 465° C. over a one-half hour time period, andthe substrate holder was continuously rotated. The temperature duringdeposition was monitored with a chromel-alumel thermocouple placed incontact with the upper side of the aluminum substrate holder of theevaporator unit. Electron beam evaporation of the materials in thecrucibles proceeded in the following order: About 150 Angstroms ofvanadium phosphide was deposited on the back face of the basesemiconductor; approximately 1,000 Angstroms of titanium was thendeposited on the vanadium phosphide, and about 500 Angstroms of platinumwas deposited on the titanium. The thickness of these layers wasmeasured by a vibrating quartz crystal monitor which had been calibratedby deposition of layers and measurement of layer thicknesses from anAuger ion beam depth profiling experiment. Therefore, all thicknessfigures in the examples are expressed as monitor readings which areapproximately one half the actual thickness. After deposition, the heatwas turned off, and the silicon crystal was allowed to cool to roomtemperature in the evaporation unit under vacuum at 10⁻⁴ millibarspressure. The crystal with the deposited layers thereon was taken out ofthe evaporation unit, and the polished face was etched for four minuteswith the previously mentioned HF-ammonium fluoride etching solution. Thecrystal was then washed with deionized water for 30 seconds on bothsides and dried with a flowing nitrogen gas stream at ambientconditions. The crystall was then all owed to sit in air for 25 minutesto grow an oxide layer before loading again into the evaporation unit.

Depositions were carried out on the front face of the crystal by placingthe crystal on the aluminum substrate holder, back-face down, and theevaporation unit was pumped down to 10⁻⁵ millibars pressure. An aluminacrucible contained a pressed pellet of SnO₂, and electron beamevaporation of the SnO₂ from the material in the carbon crucible gave athickness of about 100 Angstroms of SnO₂ on the front face of the basesemiconductor crystal. After evaporation the crystal was removed fromthe evaporation unit, placed in an aluminum oxide boat, and the assemblywas placed in a quartz tube. The quartz tube was pushed into a Lindbergtube furnace wherein the temperature was raised to 338° C. in 15 minutesin an air atmosphere, and the specimen was allowed to anneal for 11/2hours. The furnace was turned off, and when the furnace was at 100° C.,the quartz tube and sample were lifted out of the furnace and allowed tocool to room temperature. A control deposit of SnO₂ on a glass slide wasalso annealed along with the crystal, and after annealing theresistivity had dropped significantly, and the film had becomesubstantially transparent. The crystal was then returned to theevaporation unit, and the unit pumped down to a pressure of 10⁻⁵millibars pressure.

The aluminum holder in the evaporator was heated to a temperature of100° C. over a 15 minute interval, and platinum was electron beamevaporated onto the specimen from a platinum slug held in a carboncrucible. The platinum was deposited to a thickness of about 8 Angstromsat a rate of 0.2 Angstroms per second onto the SnO₂ layer which was onthe base semiconductor. Platinum was also deposited on a control glassslide; and a resistivity value of 1,880 microohm centimeters wasdetermined for the Pt layer by two-point probe techniques using indiumstrip contacts. The light transmission was also measured for theplatinum on the glass slide and a transmission of 87.8 percent wasdetermined by comparing the known amount of unattenuated light incidenton a standardized silicon solar cell used as a detection device.

After platinum evaporation onto the crystal the edges of this siliconcrystal were chopped off using a straight edge placed against thecrystal surface and striking the straight edge with a hammer. Theremoval of crystal edge material avoided a short between the front andback face. A 28 gauge silver coated copper wire was then attached to theback face of the photoelectrode by placing high purity silver paste onthe face and attaching the wire to the silver paste. The resultingphotoelectrode was placed onto a glass slide with the polished face ofthe silicon crystal facing up, and Dow Corning 100 percent blacksilicone rubber was used to seal the cell to the glass slide. The edgesof the crystal were overlapped with sealant to prevent redox-electrolytepenetration. The cell was placed in a 0.02 molar (5 gram per liter) I₂,0.4 molar (60 grams per liter) NaI, 0.5 molar KCl (37 grams per liter)aqueous solution and was tested at 1 sun illumination. A standard of 1sun illumination was determined by calibrating a solar cell obtainedfrom Allied Chemical Corporation. This cell was calibrated against acell obtained from Solarex Corporation having a known solar cellsensitivity. An EKE projector lamp was placed in an Ealing fiber opticslight source unit, and a 1/8 inch optical fiber outlet was made to a 1inch focal point lens. This system gave a parallel beam of lightincident upon the subject photoelectrode in a beaker which contained aquartz window through which the light passed to the photoelectrode. Thesolution in the beaker was mechanically stirred, and the cell wasfastened to the side of the beaker and positioned 0.5 centimeters fromthe quartz window through which the light was passed.

The short circuit current was measured with minimum resistance in thecircuit by means of an ammeter with less than 1 ohm internal resistance.The open circuit voltage was measured with a voltmeter having 10⁷ ohmsinternal resistance. The shunt resistance was measured under reversebiased conditions between 0 and -0.6 volts, or lesser volts, whereverthere was a suitable linear range. The series resistance was measured atforward bias at the linear part of the current voltage curve with thebias typically being 0.3 volts, or approximately near open circuitvoltage. The efficiencies were calculated by taking voltage and currentvalues at the tangent parallel to the line joining the open circuitvoltage and short circuit current. The product of this set of values ofvoltage and current divided by the incident power yield the efficiency.The fill factor is the product of the voltage and current at the tangentparallel to the line joining the open circuit voltage and short circuitcurrent divided by the product of the open circuit voltage and the shortcircuit current.

At 1 sun illumination various characteristic parameters were measured.The area of the cell was 0.32 cm². A short circuit current of 4.8milliamps and an open circuit voltage of 0.32 volts were determined. Ashunt resistance of 3.5 k ohms was measured from the reverse biascharacteristics. The resulting efficiency was 1.3 percent and the fillfactor was 0.24. The cell ran for a total of 51 (and still running attime of application date) complete days under 1 sun illumination in theiodide/iodine solution.

EXAMPLE 2

The procedure of Example 1 was repeated to produce an ohmic contact on awafer of resistivity 8 to 25 ohm cm, except the temperature duringdeposition of the layers was 435° C. The front face was etched as inExample 1 and was then washed with 0.01 molar tin chloride solution (tinchloride from Purtronic Corporation) for 20 seconds, washed withdeionized water, and dried with a nitrogen gas stream. The specimen wasthen oxidized in air for 25 minutes, placed in the Balzers evaporationunit, pumped down to 10⁻⁵ millibars pressure, and heated to 112° C. overa one-half hour time period. A layer of platinum of about 8 Angstromsthickness was deposited over a 30 second time period on the front faceof the photoelectrode. The resistivity of an accompanying platinum filmevaporated on a glass slide was 3,500 microohm centimeters with a lighttransmissivity of 76.5 percent. The specimen area under illumination was0.2 cm². The short circuit current was 21.7 milliamps/cm², the opencircuit voltage was 0.36 volts, the efficiency was 2.6 percent, and thefill factor was 0.30. The cell lasted for a total of approximately sixdays in the iodide/iodine solution at 1 sun illumination after which thedevice deteriorated rapidly.

EXAMPLE 3

Example 3 was prepared as Example 2 except the front face of thesemiconductor was oxidized for 3 minutes in air after the etchingtreatment. The specimen was placed in the evaporation unit, pumped to apressure of 10⁻⁵ millibars pressure, and the specimen held at atemperature of 92° C. while Pt was electron beam evaporated onto thefront face to a thickness of about 10 Angstroms over a 5 minute timeperiod, after which the specimen was cooled to room temperature invacuum. An accompanying glass slide was coated with platinum, and aresistivity of 1,300 microohm centimeters and a transmissivity of 68.5percent were measured. At 1 sun illumination and an illuminated specimensurface area of 0.12 cm² the short circuit current was 2.8 milliamps,the open circuit voltage was 0.34 volts. After 13/4 months in solutionwith occasional testing, the short circuit current decreased to 1.0milliamps, and the open circuit voltage was 0.35 volts.

EXAMPLE 4

In this example an n-type silicon single crystal had a resistivity of 6to 12 ohm centimeters, and was phosphorous doped. The back unpolishedface of the single crystal was etched 30 minutes with a 10 percent HF/36percent NH₄ F solution and washed with deionized water. The crystal wasplaced in the evaporation unit with the back face exposed, and the unitwas evacuated to 10⁻⁴ millibars pressure. Lanthanum hexaboride waselectron beam evaporated from a target pellet, and a LaB₆ layer ofapproximately 1,000 Angstroms thickness was deposited. The substrateholder was not rotated during deposition. Next, a layer of about 5,000Angstroms of platinum was electron beam evaporated onto the lanthanumhexaboride layer. The specimen was removed, the front polished face wasetched for 1 minute with 10 percent HF,/36 percent NH₄ F solution,washed with deionized water, dried in a nitrogen gas stream, and allowedto sit in air for 20 minutes. The front edges of the crystal were thenmasked off with tape, and the silicon crystal was placed in theevaporation unit with the front face exposed. The evaporation unit waspumped down to 10⁻⁴ millibars pressure, and about 200 Angstroms ofplatinum was deposited on the front face of the crystal. After 15minutes the specimen was taken out, and the edges were trimmed and aback contact with silver paste and copper wire was made as in Example 1.The front face of the cell was sealed to the glass slide with Devcon 5minute epoxy. The cell underwent illumination and testing in a glassbeaker using a 11/4 inch optical fiber and an Ealing fiber optic lightsystem to illuminate the front face of the photoelectrode. After 35continuous days in the iodide solution used in Example 1, and at anillumination of 1.5 suns, the epoxy had deteriorated so the specimen wasthen removed from solution, washed with distilled water and epoxiedagain around the edges where the old epoxy had cracked. The area of theillumination was originally 0.5 cm², and after reepoxying the area was0.25 cm². After 59 days the epoxy cracked again. The specimen wasresealed using Dow Corning 100 percent silicone rubber caulk (white) andwas retested under the same illumination conditions. At the beginning ofthe cell lifetime the illumination area was 0.5 cm² and the cell had anoutput current of 4.7 milliamps and an open circuit voltage of 0.32volts. The short circuit current was 6.0 milliamps after 43 days with anillumination area of 0.5 cm². After 61 days the cell had an output of1.7 milliamps current and 0.31 volts with an illumination area of 0.25cm². After 278 days the current was 1.1 milliamps, after 316 days it haddecreased to 0.30 milliamps, and after 346 days the output was 0.03milliamps and 0.34 volts. The efficiency was determined to beapproximately 1 percent for this cell throughout the first 273 days ofoperation.

EXAMPLE 5

This specimen received the same back face treatment as Example 1. Afterdeposition of the back face ohmic contact, the front face was etched twotimes with 10 percent HF/36 percent NH₄ F solution for 2 minutes, andthen was washed with an aluminum containing solution prepared asfollows: a solution of 10 milliliters ethanol and one drop ofAl-S-butoxide (95 percent) was mixed and centrifuged, and thesupernatant liquid was decanted, to which was added an additional 10milliliters ethanol. Approximately 1 milliliter of this solution wasadded to another 10 milliliters ethanol and used to wash the crystal forapproximately 30 seconds, and the solution was evaporated with a streamof nitrogen gas. The specimen was allowed to sit in air for 25 minutesto form a doped oxide layer and then was placed in the evaporation unitwith the front face of the device exposed. The evaporation unit waspumped down to 10⁻⁵ millibars pressure and heating was commenced. Whenat 100° C. for about one-half an hour, platinum was electron beamdeposited as in Example 1. A total of about 8 Angstroms platinum wasdeposited over a period of 30 seconds, the heat was turned off, and thespecimen allowed to cool to room temperature over one half hour timeperiod. The accompanying glass slide with platinum deposited thereonshowed a platinum film resistivity of 3,190 microohm centimeters and84.4 percent transmissivity of light. The specimen was then made into aphotoelectrode by attaching silver paste and a copper wire as inExample 1. The specimen was placed in the Lindberg tube furnace with ahydrogen atmosphere passed over the specimen which was heated to atemperature of 200° C. as measured by a chromel-alumel thermocoupleadjacent the specimen. The temperature was held at 200° C., and ahydrogen gas atmosphere was maintained over the specimen for 16 hours,after which the specimen was allowed to cool in the oven to roomtemperature. The specimen was placed in the photoelectrochemical cellsolution with the electrolyte solution of the same composition as inExample 1. At 1 sun illumination the output was 6.8 milliamps and 0.42volts for an area of 0.35 cm². Subsequently, the specimen was tested ina concentrated I₂ /I⁻ solution with the maximum amount of iodine insolution. At 1/2 sun illumination a voltage of 0.24 volts was measuredand a current output of 4.4 milliamps was determined for an area ofillumination of 0.35 cm². The efficiency was determined to be 6.6percent, the series resistance was approximately 23 ohms and the shuntresistance was greater than 10 k ohms.

EXAMPLE 6

The base silicon semiconductor had a resistivity of 8 to as much as 25ohm cm. An ohmic contact was made as in Example 1. The front face ofthis specimen was swabbed with a dry Q-tip to remove dust particles andthen allowed to oxidize a period of 10 minutes in air after carrying outthe same etching operation of Example 1. Ten Angstroms of Pt wereelectron-beam deposited on the front face at 0.02 Angstroms per secondat 110° C. and 10⁻⁶ mbar pressure. A control slide gave a Pt resistivityof 580 microohm and a transmissivity of 69 percent. This specimen wasplaced in an aluminum oxide boat, placed in the Lindberg tube furnace,and purged with a 1 percent H₂ /Ar atmosphere for a period of 15minutes. The specimen was heated over a one hour period to 300° C. leftfor a period of 16 hours at 300° C. and cooled to room temperature over2 hours, purging continuously with the H₂ /Ar gas atmosphere. Thespecimen was sealed with the Dow Corning silicone rubber as in Example4, and the resulting photoelectrode was immersed in a solution of 1molar sodium iodide, 0.1 molar I₂, and 0.2 molar KCl in hydrochloricacid such that the pH of the solution was unity. The specimen was thenilluminated as in previous examples by a parallel light beam, and theshort circuit current was 0.18 milliamps, and the open circuit voltagewas 0.40 volts.

EXAMPLE 7

The preparation of Example 7 was the same as Example 6, except fourdifferent times of air oxidation of the front face after etching wereemployed: 2 minutes, 10 minutes, 25 minutes, and 50 minutes. Thesespecimens were put in the evaporation unit, pumped to a pressure of 10⁻⁵millibars pressure, and were heated for a period of one-half hour to110° C. Each of these four specimens had about 10 Angstroms of platinumdeposited at a rate of 0.02 Angstroms per second, reaching a totalthickness of about 10 Angstroms of platinum. The accompanying glassslide with platinum deposited thereon showed a resistivity of 580microohm centimeters and a 60.9 percent transmissivity of light. Thesamples were tested at 1 sun illumination in the same iodide-iodinesolution used in Example 6. The four specimens subjected to differenttimes of oxidation showed the following short circuit currents and opencircuit voltages: for 2 minutes oxidation the short circuit current was22 milliamps/cm.sup. 2 and 0.36 volts open circuit voltage with an areaof illumination of 0.12 cm². For the specimen subjected to 10 minutesair oxidation the short circuit current was 27 milliamps/cm² and an opencircuit voltage of 0.36 volts with an area of 0.21 cm². For the specimensubjected to 25 minutes oxidation the short circuit current was 27.5milliamps/cm² and the open circuit voltage was 0.36 volts for an area of0.16 cm². The specimen oxidized for 50 minutes showed a short circuitcurrent of 7 milliamps/cm² and an open circuit voltage of 0.35 voltswith an area of 0.18 cm². The efficiency of the specimen oxidized for 25minutes was approximately 3 per cent.

EXAMPLE 8

The preparation of the back contact of this Example was the same asExample 1, and the preparation of the front face was the same as Example7, except three specimens were prepared with oxidations lasting for 2minutes, 10 minutes, and 25 minutes. These three specimens were thentransferred to the evaporation unit and pumped down to a pressure of10⁻⁵ millibars pressure, heated to 100° C. over a period of one-halfhour, and 10 Angstroms of platinum were deposited on each of thespecimens at a rate of about 0.25 Angstroms per second. The glass slidewith the accompanying platinum film showed a resistivity of 440 microohmcentimeters and a light transmissivity of 66.1 per cent. These threespecimens were made into photoelectrodes, immersed in the iodide-iodinesolution used in Example 7, and tested at 1 sun illumination. Thespecimen oxidized for 2 minutes showed a short circuit current of 20.0milliamps/cm² with an open circuit voltage of 0.37 volts for an area of0.08 cm². The specimen oxidized for 10 minutes showed a short circuitcurrent of 24.2 milliamps/cm² with an open circuit voltage of 0.37 voltsand an area of 0.12 cm². The specimen oxidized for 25 minutes showed ashort circuit current of 26.5 milliamps/cm² and an open circuit voltageof 0.37 volts with an area of 0.20 cm².

EXAMPLE 9

This specimen was prepared the same as Example 5, except hydrogen gaswas brought into the Balzers evaporation unit at a pressure of 0.1atmosphere, and the specimen was annealed to 385° C. for one-half hour,the heating unit turned off, and the specimen allowed to cool to 100° C.over a one hour time period. The specimen chamber was then pumped downto 10⁻⁵ millibars pressure and 8 Angstroms of platinum were deposited onthe front face of the semiconductor. The accompanying glass slide with aplatinum film deposited thereon showed a resistivity of 3,050 micro-ohmcentimeters and a 78.1 per cent light transmissivity. The edges of thecell were cut off as in Example 1, and a contact made to the front faceusing a thin wire and silver paste, but leaving a large area uncoveredfor illumination. The contact was covered with sealant, and the specimenwas immersed in the iodide-iodine solution used in Example 1. With anarea of 0.25 cm² the photoelectrode front face was illuminated at 1 sunillumination. A short circuit current of 5.9 milliamps, an open circuitvoltage of 0.34 volts, and an efficiency of 3 percent were measured. Theseries resistance was 51 ohms, and the shunt resistance wasapproximately 2 k ohms. The cell was then taken out of solution,illuminated in air, and the same parameters measured. With 1 sunillumination from an optic fiber, of 0.07 cm² area, up against the frontface of the cell, a short circuit current of 0.17 milliamps and an opencircuit voltage of 0.18 volts were measured. The series resistance was20 ohms, and the shunt resistance was 2 k ohms.

EXAMPLE 10

An n-type silicon wafer of 8 to 25 ohm cm resistivity was used in thisexample. The back face ohmic contact was prepared by a well-known methodusing an Emulsitone phosphorosilica film: the Emulsitone phosphorosilicafilm was lightly brushed onto the center of the back face of thecrystal, allowed to dry in air, and then placed in a ceramic boat. Thespecimen was heated in a large Lindberg-type tube furnace using Ar/N₂mixture as a purge gas. The specimen was gradually pushed in so as to beat 100° C. for 5 minutes, and then at 800° C. for 1 minute, and thenheld at 1,075° C. for 15 minutes. The specimen was removed from thefurnace over a period of onehalf hour, and the back face was etched 5minutes in 10 volume per cent HF/36 volume per cent NH₄ F aqueoussolution. The solution was changed three times during that 5 minute timeperiod. The specimen was washed in nanopure deionized water supplied bySybror/Barnstead with the water resistivity about 8 to 12 megaohmcentimeters. The crystal remained in air for a total of 45 minutes, andthe crystal was re-etched on the back face for 30 seconds with theHF-NH₄ F solution, washed with deionized water for 45 seconds, and thenwashed with nanopure water until the surface conductivity of thespecimen stabilized. The specimen was dried with nitrogen gas, placed inthe evaporation unit, and pumped to 10⁻⁵ millibars pressure.Approximately 1,000 Angstroms of titanium were deposited on thespecimen, followed by deposition of about 1,000 Angstroms of platinum.The specimen was removed to air for 15 minutes, etched with the HF-NH₄ Fsolution for 30 seconds, washed with deionized water for 45 seconds,followed by a nanopure water washing, and drying with nitrogen gas.After this washing operation, the specimen was immediately placed in theevaporation unit and approximately 17 Angstroms of SiO₂ were depositedby electron beam evaporation from a silicon dioxide pellet. Over a 10minute time period 100 Angstroms of platinum were deposited on thesilicon oxide layer while maintaining the specimen at 150° C. Thephotoelectrode was made up and tested in air as in Example 9. With 1 sunillumination on an active area of 0.07 cm², the short circuit currentwas 0.46 milliamps, and the open circuit voltage was 0.21 volts. Theseries resistance was 3.3 ohms, and the shunt resistance was 840 ohms.The specimen of area 0.5 cm⁵ was immersed in the iodide-iodine solutionused in Example 1, and illuminated at 1 sun intensity. The short circuitcurrent was 5.1 milliamps, and the open circuit voltage was 0.42 volts.The series resistance was 26 ohms, and the shunt resistance was 2 kohms.

EXAMPLE 11

In this Example three specimens with different base semiconductorresistivity were tested, and preparation and testing were the same asExample 2. The first specimen had a resistivity of 1.0 ohm centimeters,the second a resistivity of 0.1 ohm centimeters, and the third had aresistivity of 8 to 25 ohm centimeters. The first photoelectrodespecimen had an illumination area of 0.45 cm². A short circuit currentof 9.1 milliamps and open circuit voltage of 0.34 volts was determined.The second specimen with an area of 0.36 cm² had a short circuit currentof 7.1 milliamps and an open circuit voltage of 0.35 volts. The shortcircuit current density was 19.7 milliamps/cm², and the saturatedcurrent density was 22.8 milliamps/cm². The third specimen with 8 to 25ohm centimeters resistivity, with an area of 0.24 cm², had a shortcircuit current of 5.2 milliamps and an open circuit voltage of 3.6volts.

EXAMPLE 12

A silicon crystal the same as used in Example 2 was used in thisExample. The back face contact consisted of the same type of contact asprepared for Example 1, with about 150 Angstroms of vanadium phosphidedeposited on the back face of the semiconductor, followed by about 1,000Angstroms of titanium, and then a final layer of about 500 Angstroms ofplatinum with deposition carried out at 435° C. The front face of thespecimen was etched 4 minutes with the HF-NH₄ F solution as in Example1, washed with deionized water for 30 seconds, and dried with nitrogengas, and immediately placed in the evaporator with a blank silicon waferplaced over the front face for protection. The sample was heated to 465°C. at 10⁻⁴ mbar pressure for 30 minutes to oxidize the front face at thesame time as an ohmic contact was deposited on another wafer. Thespecimen was replaced in the evaporation unit, pumped down to 10⁻⁵millibars pressure, and heated to 112° C. over a one-half hour timeperiod. Approximately 8 Angstroms of platinum were deposited in a 40second time period. The cell was prepared and tested as in Example 1.The device was subjected to 1 sun illumination with a 1/4 inch fiberoptic placed against the beaker, and the cell had an illumination areaof 0.45 cm². A short circuit current of 8.3 milliamps and an opencircuit voltage of 0.35 volts were measured.

EXAMPLE 13

In this example, the specimen was prepared from a p-type, 1.0 ohmcentimeter resistivity, single-crystal silicon wafer. A back face ohmiccontact was prepared by depositing about 1,000 Angstroms of aluminum andabout 1,000 Angstroms of gold sequentially by electron beam evaporationfrom material in carbon crucibles. The deposited aluminum acted to dopethe silicon layer to make a p+ doped layer. The specimen was removed,the front face was etched for 30 seconds in HF-NH₄ F solution, washedwith deionized water for 30 seconds, and dried with a flowing nitrogengas stream. The specimen was placed in a ceramic boat and the entireassembly placed into a quartz tube in a tube furnace held at 576° C. Theceramic boat containing the silicon specimen was moved into the tubefurnace over a 3 minute time period; oxygen gas was passed over thespecimen for a period of 3 hours at a temperature of 576° C. in thefurnace, and nitrogen gas was turned on as the oxygen gas stream wasturned off. The temperature was allowed to decrease to 200° C. over aone hour time period, the furnace was opened, and the ceramic boat andspecimen were removed from the furnace to cool to room temperature. Aportion of this sample was analyzed by Auger spectroscopy and a layer ofapproximately 6 Angstroms of silicon oxide was measured as a consequenceof the oxygen treatment at 576° C. The specimen was placed into theevaporation unit, pumped to 10⁻³ millibars pressure, oxygen gas let intothe evaporation unit to a pressure of approximately 1 atmosphere, andthen the unit was pumped to 10⁻⁵ millibars pressure. At that point 30Angstroms chromium were electron beam deposited onto the front face ofthe specimen, and about 8 Angstroms of platinum was electron beamdeposited over the chromium layer. During deposition of the platinum,the specimen was held at a temperature of 112° C., and after depositionof the platinum, the specimen was allowed to cool to room temperature.This treated specimen was made into a photoelectrochemical cell andtested, as in Example 9, with an illumination area of 0.12 cm². Theshort circuit current was determined to be 0.30 milliamps or 2.5milliamps/cm² and the open circuit voltage was 0.34 volts. Measurementswere made in air for the same area, and a short circuit current of 0.70milliamps was measured or a 5.8 milliamps/cm² current density with anopen circuit voltage of 0.30 volts.

EXAMPLE 14

The crystal used in this example had a p-type resistivity of 0.1 ohmcentimeters. The ohmic contact was made and the front face etched andoxidized as in Example 13. The specimen was then placed front face up inthe evaporation unit, the evaporation unit was pumped to a pressure of10⁻⁵ millibars pressure, and 200 Angstroms of SnO₂ were electron beamdeposited at a rate of 1 Angstrom per second. The specimen was removedfrom the evaporation unit and placed in a ceramic boat. The specimen inthe boat was heated in a Lindberg tube furnace in air to a temperatureof 356° C. to partially oxidize the SnO₂. The specimen was held at atemperature of 356° C. in an air atmosphere for 11/2 hours and was takendirectly out of the furnace to cool to room temperature. The specimenwas sealed in the photocell as in Example 9, and the measurements werecarried out at 1 sun illumination with an area of illumination of 0.35cm². In air the short circuit current was 3.6 milliamps and the opencircuit voltage was 0.12 volts. Measurements were also done in anelectrolyte of S²⁻ /S_(n) ²⁻ (polymeric) solution at 1 sun illuminationfor an area of 0.35 cm². There was virtually 0 milliamp short circuitcurrent and 0.28 volts open circuit voltage. In dilute iodine/iodidesolution the open circuit voltage was 0.11 volts and the short circuitcurrent was 0 milliamps. A glass slide was also deposited with SnO₂ andbefore annealing in the Lindberg tube furnace and air at 356° C., theresistivity was 8×10⁻² ohm centimeters with an 85.1 per cent lighttransmissivity. After annealing in air the resistivity was 4.4×10⁻³ ohmcentimeters and the light transmissivity was 87.3 per cent.

EXAMPLE 15

An 8 to 25 ohm resistivity n-silicon wafer was etched on the front facefor 10 sec. with a 10 per cent HF, 36 per cent NH₄ F solution, washedwith deionized water and dried in a stream of nitorgen. The back sidewas then etched with the above solution for 1 minute, washed and driedas above, and brushed with a phosphorosilica solution, as used inExample 10, diluted 3 times with ethanol. The wafer was dried in air at100° C. for 2 minutes, and heated in a water saturated stream ofnitrogen for 30 minutes at 450° C., and moved out of the tube furnaceover 5 minutes. The front face of the specimen was then etched 30seconds with HF-NH₄ F solution, washed with deionized water, and driedwith a flowing nitrogen gas stream. The specimen was put in theevaporation unit, pumped to a pressure of 10⁻⁵ millibars pressure, andheated to a temperature of 150° C. Over a 10 minute time period 100Angstroms of platinum were deposited on the front face. The back facewas etched for 7 minutes with HF-NH₄ F solution, washed with deionizedwater and dried in nitrogen. After placing in the evaporator, 1,000Angstroms Pt were deposited on the back face. After preparation andtesting in the same manner as in Example 9, the short circuit currentwas measured to be 0.34 milliamps, and the open circuit voltage was 0.21volts in air. The series resistance was 11.5 ohms, and the shuntresistance was 1.3 k ohms. Tests were done with 1 sun illumination and a1/4 inch fiber optic device was placed against the beaker containing theelectrolyte and cell. The short circuit current was 2.4 milliamps, andthe open circuit voltage was 0.3 volts with an illumination area of 0.27cm². The series resistance was 9.5 ohms and the shunt resistance wasgreater than 10 k ohms.

EXAMPLE 16

An ohmic contact was made on both the front and back surfaces of ann-type, phosphorous doped single crystal of 8-25 ohm centimetersresistivity. The back unpolished face was etched for 2 minutes with asolution containing 10% HF/36% NH₄ F, the specimen was washed withdeionized water, dried in a stream of nitrogen, and mounted in theelectron beam evaporator with a 6.7 to 12 ohm cm n-silicon wafer againstthe front fact to prevent contamination of the front face. The samplewas loaded into the evaporation chamber and was pumped down toapproximately 10⁻⁵ millibars pressure. The substrate was heated to 440°C. over 30 min, and held at 440° C. for 20 minutes. Vanadium phosphideof thickness 150Å was electron beam deposited over a 5 minute period.The aluminum substrate holder was rotated during evaporation. Thevanadium phosphide target was a large pressed pellet placed in a carboncrucible for evaporation. Between runs the VP pellet was stored undernitrogen. Next, 1000Å titanium was deposited over 1.5 minutes timeperiod. 500Å of platinum was then deposited over 8 minutes. The heat wasthen turned off, and the substrate holder cooled to 127° C. overapproximately 1 hour. Nitrogen was let into the chamber, and the sampleremoved approximately 1 hour later.

The contact on the polished front face was made as follows: the frontface was etched for 4 minutes in the HF/NH₄ F solution, washed well indeionized water, and dried in a stream of nitrogen gas. The 150Åvanadium phosphide, 1000Å Ti, and 500Å Pt were deposited under theconditions given above.

All the edges of the wafer were trimmed and electrical connections weremade to the back and front face by connecting flattened zig-zag sectionsof 20 gauge silver-coated copper wire using silver paste (SPI #5001).The connections were then coated over with Devcon epoxy. The DCresistance was 2.3 ohm with the back face positive, and 2.2 ohm with thefront face positive. The current versus voltage plot showed an almostlinear response over the voltage range ±40 mV. With the front facenegative, the response was almost linear up to at least 0.35 V, giving aresistance of 2.6 ohm.

EXAMPLE 17

A silicon n-type crystal as in Example 16, was etched on the back facefor approximately 20 min. in 10% HF, 36% NH₄ F solution, washed withdeionized water for about 30 seconds, and dried in air. Then, about1000Å of lanthanum hexaboride (99% Alfa Chemicals) and about 5000Åplatinum (Marz grade, MRC) were e-beam deposited at about 10⁻⁴ millibarspressure in the evaporation unit. The substrate holder was not rotatedor heated.

The polished front face was etched for 1 minute with 10% HF, 36% NH₄ Fsolution, washed with deionized water for about 30 seconds, and dried inair. After immediately placing the wafer in the evaporator and pumpingthe chamber to 10⁻⁴ millibars pressure, 20Å SiO₂ was electron beamdeposited from a crushed quartz tube in a carbon crucible, followed by200Å of Pt, both deposited with an unheated aluminum substrate holder.

Electrical connections were made to both the back and front face as inExample 9, and the whole cell, apart from a portion of the front face,was sealed with Devcon 5-min. epoxy. The photoelectrode was tested inthe following 3 configurations: (1) The photoelectrode was in air andthe two contacts on the photoelectrode, front and back face, wereconnected to two platinized platinum electrodes both immersed in thesame redox solution. The voltage was measured between the back facecontact of the photoelectrode and one of the platinum electrodes; (2)The entire photoelectrode was immersed in the redox solution, and thevoltage was measured between the front and back contacts of thephotoelectrode; (3) The entire photoelectrode was immersed in the redoxsolution but only the back contact was utilized. This was joined, via avoltmeter, to a platinized platinum counter electrode placed in the sameredox solution. Three different redox solutions were used: (1) 0.04MBr₂, 0.4M NaBr, 0.4M KCl; (2) 0.02M I₂, 0.4M NaI, 0.5M KCl; and (3) 1MFe(EDTA)Cl₂, 1M Fe(EDTA)Cl, all made up with deionized water. The redoxpotentials (in volts) of the solutions were measured with an in situstandard calomel electrode using a Keithley electrometer. The followingtable shows open circuit voltages obtained for the 3 configurationsdescribed above.

    ______________________________________                                        Redox      Redox    1 sun illumination                                        Solution   Potential                                                                              V.sub.1    V.sub.2                                                                            V.sub.3                                   ______________________________________                                        Br.sub.2 /Br.sup.-                                                                       +0.84    0.30       0.42 0.44                                      I.sub.2 /I.sup.-                                                                         +0.31    0.30       0.32 0.30                                      Fe.sup.3+ /Fe.sup.2+                                                                     -0.20    0.33       0.27 0.28                                      ______________________________________                                    

EXAMPLE 18

An n-type silicon crystal as in Example 17 had an ohmic contact preparedas in Example 1. The front face was swabbed with a Q-tip, was etchedwith 10% HF, 36% NH₄ F solution twice for two minutes, washed indeionized water, dried in a stream of nitrogen gas, and immediatelyplaced in the evaporation unit, which was pumped down to 10⁻⁴ millibarspressure. The sample was heated to 110° C. over a 30 minute time, andabout 8 Angstroms of tantalum were deposited at a rate of 0.08 Angstromper second at 110° C., followed by depositing about 10 Angstrom platinumdeposited at about 0.17 Angstrom per second. The substrate was cooled to60° C., nitrogen gas was let into the chamber, and the samples removedwhen the temperature reached ambient. Under the conditions ofevaporation using a very slow rate of deposition in a poor vacuum, thetantalum oxidized to a film of tantalum oxide.

Both back and front face contacts were made as in Examples 1 and 9 butusing Devcon 5-minute epoxy, leaving 0.45 cm² of the front face exposedfor illumination. At one sun illumination in air, the open circuitvoltage was 0.41 volts, and the short circuit current was 2.9 mA. At onesun illumination in the dilute iodine-iodide solution of Example 1, theopen circuit voltage was 0.45 volts, and the short circuit current 5.4mA. In a dilute bromide-bromine solution having emf verses standardcalomel emf of 0.77 volt, and at one sun illumination, the open circuitvoltage was 0.52 volt and the short circuit current was 1.7 mA. In thesulfide/polysulfide solution, having emf versus standard calomel emf ofminus 0.71 volts, at one sun illumination, the open circuit voltage was0.06 volt, and the short circuit current was 0.01 mA. Under 0.69 sunillumination, using a concentrated I₂ /I⁻ solution, having an emf versusstandard calomel of 0.33 volts, the open circuit voltage was 0.37 volt,and the short circuit current was 4.0 mA. Under 0.69 sun illumination,using a concentrated Br₂ /Br⁻ solution of emf versus standard calomelemf of 0.79volts, the open circuit voltage was 0.50 volt, the shortcircuit current was 4.0 mA, and the efficiency was about 5 per cent. Forall the above measurements, the short circuit currents and open circuitvoltages were 0 in the dark.

EXAMPLE 19

The crystal was the same as Example 1, and the ohmic contact was made asin Example 1. The front faces of the three crystals were etched in 24%HF solution twice each for 5 minutes, washed in deionized water, anddried in a stream of flowing nitrogen gas. Samples A, B, and C, wereleft to oxidize in air for one hour, 25 minutes, and two minutes,respectively, were placed in the evaporation unit, pumped to 2×10⁻⁶millibars pressure, and about 20 Angstrom of platinum were deposited(without sample heating) at a rate of 0.13 Angstrom per second. Afterdeposition, the samples were heated to 330° C. in vacuum over a timeperiod of about 30 minutes, left 5 minutes at 330° C., cooled to 200° C.in 15 minutes, nitrogen was let into the chamber, and the samples cooledto room temperature. The cells were prepared and tested in aniodine-iodide solution as in Example 1 at one sun illumination. Theilluminated areas, short circuit currents, and open circuit voltageswere as follows: cell A; 0.20 cm², 3.1 mA, 0.32V; cell B, 0.21 cm², 3.0mA, 0.30V; cell C, 0.32 cm², 6.2 mA and 0.36V. Cells A and B continuedto function with approximately the above outputs after 25 dayscontinuous running at one sun illumination in iodine-iodide solution.Cell C ceased to function after two hours under the same conditions.

EXAMPLE 20

The back of a one ohm cm resistivity, p-type silicon crystal was etchedfor 3 minutes in 10% HF, 36% NH₄ F solution, and washed in deionizedwater. Five thousand Angstroms of aluminum was deposited on the samplein the evaporation unit after pumping to the 10⁻⁴ millibars pressure.The front face was etched for 30 seconds with 10% HF, 36% NH₄ F, washedwith deionized water, dried in a stream of nitrogen gas, placed in aLindberg tube furnace in an aluminum boat, heated to 580° C. for 8.5hours, and cooled in the furnace to room temperature. The edges of thecrystals were masked with masking tape prior to placing in theevaporation unit and pumped down to 10⁻⁴ millibars pressure. The samplewas heated to 320° C. on the rotating substrate holder, and 100Angstroms of platinum was deposited.

The front face of the crystal was examined by Auger electronspectroscopy while argon-ion etching. Peaks from platinum, oxygen,silicon, and carbon were monitored as a function of depth from thesurface. The main features were consistent with, as a function of depth,a platinum layer with virtually no oxygen, then a silicon dioxide layer(which was confirmed by the presence of silicon in a +4 valence state),followed by the silicon base semiconductor. A minor feature at theimmediate surface indicates the presence of a small proportion ofsilicon dioxide over the platinum layer.

EXAMPLE 21

An n-type silicon crystal of 1.0 ohm centimeter resistivity was preparedas in Example 18. After etching and washing, the crystal was immediatelyplaced in the evaporator and pumped down to 10⁻⁴ mbar pressure. Thesample was heated to 100° C. and 8 Angstroms of yttrium doped ZrO₂ wasdeposited by electron beam evaporation from a pellet pressed from ZrO₂powder doped with 8 per cent Y₂ O₃ placed in an aluminum oxide crucible.Deposition was performed at about 0.08 Angstroms/second at 5×10⁻³ mbarpressure. The silicon crystal was heated to 200° C. during deposition of10 Angstroms of platinum at a rate of 0.09 Angstroms/second. Aftercooling to room temperature overnight the specimen was made into aphotoelectrode as in Example 9 with a cell illumination area of 0.18cm². At 1 sun illumination in air the open circuit voltage was 0.27volts, and the short circuit current was 0.17 mA. At 1 sun illuminationin the solution of Example 1 the open circuit voltage was 0.50 volts,the short circuit current was 3.8 mA, and the efficiency was about 4 percent. In a dilute bromide-bromine solution the open circuit voltage was0.56 volts, and the short circuit current was 3.6 mA. Both the opencircuit voltages and short circuit currents were zero without anyillumination.

Example 22

In this example the type of starting crystal and preparation of the backohmic contact are the same as in Example 1. The specimen was etched for4 minutes in HF/NH₄ F solution, washed with deionized water, dried in aflowing nitrogen gas stream, and exposed to air for 25 minutes underambient conditions. The specimen was placed in the evaporator unit, thechamber pumped to a pressure of 10⁻³ mbar and backfilled to 1 atmospherepressure with oxygen. The chamber was then pumped to 5×10⁻⁵ mbarpressure, and 500 Angstroms of tin oxide doped with indium were electronbeam deposited from an indium-tin oxide pellet held in an aluminacrucible. The indium-tin oxide material was obtained from EM ChemicalsCorporation. After deposition of the doped tin oxide, the crystal wasplaced in an alumina boat and positioned in the hot zone of a tubefurnace at 211° C. for 30 minutes in an air atmosphere. After thisannealing step, the resistivity of an indium doped, tin oxide filmelectron beam deposited on a glass slide was reduced from 4.5×10⁻² to7.5×10⁻⁴ ohm centimeters, and the light transmissivity was increasedfrom 21.6 to 66.6 percent.

The crystal was replaced in the evaporator unit and pumped to a pressureof 10⁻⁴ mbar, and 8 Angstroms of platinum were deposited at a crystaltemperature of about 110° C. and with a deposition rate of 0.18Angstroms/second. The photoelectrode was prepared and tested as inExample 1. At 1 sun illumination and using an iodine/iodide redox coupleas in Example 1, the short circuit current was 3.9 mA with a cell areaof 0.27 volts. The cell lasted six days voltage was 0.27 volts. The celllasted six days under continuous operation.

I claim:
 1. A process for the preparation of a multilayer structurewhich is suitable for use as either a photovoltaic cell or aphotoelectrode in a photoelectrochemical cell which comprises:(a)forming a layer of insulator material on an n-type semiconductor,wherein said insulator material is negatively charged as a consequenceof the presence of aliovalent dopant ions and has a thickness which iseffective to permit electron tunneling through it; (b) forming a layerof conducting material on said layer of insulator material; and (c)annealing the resulting structure.
 2. The process of claim 1 whichfurther includes the preparation of an ohmic contact on the back face ofsaid semiconductor comprising the steps of:(a) depositing on saidsemiconductor a first layer comprising a compound which contains atleast one first element in anionic form which is selected from the groupconsisting of P, As, and Sb, and at least one second element in cationicform which is selected from the group consisting of V, Ti, Zr, Hf, Nb,and Ta; and (b) depositing a second conducting layer on said firstlayer.
 3. The process of claim 2 wherein said first layer comprisesvanadium phosphide.
 4. The process of claim 2 wherein said second layeris selected from the group consisting of Pt, Pd, Au, W, Ta, MoSi₂, dopedSnO₂, doped oxides, and conducting organics.
 5. The process of claim 1wherein said insulator material comprises an insulator other than anoxide of said semiconductor.
 6. The process of claim 1 wherein saidconducting material comprises a noble metal.
 7. The process of claim 1wherein said conducting material is substantially transparent to light.8. The process of claim 1 wherein said conducting material consists ofplatinum.
 9. The process of claim 8 wherein said layer of conductingmaterial has a thickness in the range from about 10 to about 200Angstroms.
 10. The process of claim 1 wherein said semiconductor issilicon and said conducting material is platinum.
 11. The process ofclaim 10 wherein said layer of conducting material has a thickness inthe range from about 10 to about 50 Angstroms.
 12. The process of claim1 wherein said layer of insulator material has a thickness in the rangefrom about 10 to 25 Angstroms.